1. Field of the Invention
The present invention relates to a control device of an internal combustion engine to adjust a fuel injection quantity based on an activity of oxygen occlusion properties of a three-way catalyst after cold start of the internal combustion engine.
2. Description of the Related Art
Current gasoline automobiles have a three-way catalyst mounted as an exhaust gas purification system. In the three-way catalyst, noble metal, that is, Pt (platinum), Pd (palladium), and Rh (rhodium) are supported. This three-way catalyst functions to convert harmful gas components of an automobile (HC, NOx, CO) to be harmless gases of CO2, H2O, and N2 by the oxidation-reduction reactions, being a catalytic action. As is well known, such a relation as shown in FIG. 13 exists between an air-fuel ratio upstream from a three-way catalyst and catalytic conversion efficiency, and catalytic conversion efficiency is the highest where an air-fuel ratio is in the vicinity of a theoretical air-fuel ratio. To enhance the catalytic action, it is important to keep an exhaust gas at a theoretical air-fuel ratio. That is, with reference to FIG. 13, when controlling an air-fuel ratio in the window, the three-way catalyst acts as an effective conversion catalyst with respect to any gas. Additionally, in this diagram, HC refers to hydrocarbon, NOx nitrogen oxides, CO carbon monoxide, and V an oxygen concentration sensor output.
Further, the three-way catalyst contains therein Ceria (Ceria: cerium oxide) and the like as a promoter. Ceria or the like has properties of discharging oxygen in the case of being rich, and of absorbing oxygen in the case of being lean depending on air-fuel ratio upstream from the three-way catalyst as shown in FIG. 14. As a result, even in the case where an air-fuel ratio upstream from the three-way catalyst is varied and becomes out of a theoretical air-fuel ratio, an air-fuel ratio in the three-way catalyst comes to be a theoretical air-fuel ratio at all times, thus suppressing emissions of harmful gases. In addition, FIG. 14 shows behaviors of an air-fuel ratio upstream from a three-way catalyst, an amount of oxygen occlusion, and an air-fuel ratio in the three-way catalyst, respectively.
Furthermore, in the internal combustion engine, to regularly keep an air-fuel ratio in the vicinity of a theoretical air-fuel ratio, the air-fuel ratio feedback is carried out. In a general air-fuel ratio feedback system, an air-fuel ratio sensor (oxygen concentration sensor) is mounted at a place of an exhaust system as near to a fuel combustion chamber as possible, i.e., on the upstream side of a three-way catalyst to make the feedback control of fuel injection quantity of an engine so that combustion gas is at a theoretical air-fuel ratio. In spite of the air-fuel ratio feedback control, an air-fuel ratio is varied to be out of a theoretical air-fuel ratio at the time of acceleration/deceleration time. However, with a promoter of Ceria or the like, the internal part of a three-way catalyst is kept at a theoretical air-fuel ratio.
In this respect, at the time of acceleration of cold start of an internal combustion engine, since much fuel is adhered to a portion in the vicinity of an inlet port, thereby a fuel quantity to be sucked into a cylinder being reduced, an air-fuel ratio is generally set to be more rich than after warming-up. However, when the activity of a promoter of Ceria or the like of a three-way catalyst is not high enough, an occlusion amount of oxygen becomes insufficient, and HC conversion efficiency is reduced, eventually resulting in a disadvantage of worse HC emissions.
To cope with this, for example, the Japanese Patent Publication (unexamined) No. 157251/1987 discloses a control device consisting of control means for making such a feedback control that an air-fuel ratio of an internal combustion engine is a target air-fuel ratio, and temperature detection means for detecting a temperature of the internal combustion engine. In the case where a temperature of an internal combustion engine is not higher than a predetermined temperature, a target air-fuel ratio is set to be on the side of being more lean than a theoretical air-fuel ratio, thus suppressing worse HC emissions.
However, as to an activity of a noble metal and that of oxygen occlusion properties of a three-way catalyst after cold start of an internal combustion engine, as shown in FIG. 15, the activity of a noble metal is immediately increased, while the activity of oxygen occlusion properties are gradually increased. In the conventional air-fuel ratio control, since the air-fuel ratio is controlled to be in the vicinity of a theoretical air-fuel ratio despite that oxygen occlusion properties of a three-way catalyst is increased by degrees as shown in FIG. 12, an occlusion amount of oxygen is unlikely to increase. As a result, when an air-fuel ratio comes to be rich due to, e.g., acceleration, oxygen in the three-way catalyst is insufficient, and an air-fuel ratio in the three-way catalyst cannot be kept at a theoretical air-fuel ratio. In this respect, the inventor has noted that the emissions of HC are increased while a noble metal of the three-way catalyst being activated, as shown in FIG. 15. In addition, FIG. 12 shows problems in the conventional control, and indicates behaviors of an air-fuel ratio, oxygen occlusion properties and an amount of oxygen occlusion of a three-way catalyst, an air-fuel ratio in the three-way catalyst, and HC emissions, respectively.
Additionally, as described above, in the case where a temperature of the internal combustion engine is not higher than a predetermined temperature, just by the method of control of setting a target air-fuel ratio to be on the side of being more lean than a theoretical air-fuel ratio, no control is made considering activity transition of oxygen occlusion properties after cold start of an internal combustion engine. Thus, a problem exits in that the three-way catalyst is saturated with oxygen, and the emissions of NOx are increased.